Method and circuit arrangement for protecting an electric motor from an overload

ABSTRACT

A method and a circuit arrangement suitable for performing the method are provided for protecting an electric motor ( 10 ) and/or its trigger circuit ( 12 ) against overload in the emergency-operation mode. The method of the invention and the associated circuit arrangement serve in particular to protect a motor vehicle direct-current fan motor operated by means of pulse width modulation, in which the trigger circuit is designed as an emergency-operation controller, with a microcontroller ( 16 ) preceding the motor end stage ( 22 ) and with a comparator assembly ( 14 ) for detecting overvoltages. The provisions according to the invention assure overload protection at elevated battery voltage and at the same time when the emergency-operation controller is activated in response to malfunctions in the normal triggering ( 12 ) of the electric motor ( 10 ). To that end, at least one overvoltage threshold (U s1 , U s2 ) is defined; when it is exceeded, the power supplied to the motor is reduced or switched off.

PRIOR ART

[0001] The invention is based on a method for protecting an electricmotor and/or its trigger circuit against overload, and on a circuitarrangement for performing such a method, the method being describedgenerically by the preamble to claim 1. To that end, from German PatentDisclosure DE 199 44 194 A, an overload protector for an electronicallycommutable motor is known, whose end stage can be triggered via anelectronic control unit by means of PWM control signals and can besupplied with a supply voltage. A limitation to a maximal load withoverload protection of the motor is attained by providing that as afunction of the magnitude of the supply voltage and of the specifiedset-point value, for the PWM control signals at least from the time therated voltage of the motor is exceeded, the pulse width of the PWMcontrol signals for the end stage is reduced to values that prevent anoverload of the motor and of the electronic components by limiting themotor power. However, a circuit arrangement of this kind assuresoverload protection only as long as the triggering of the end stage byPWM signals does not itself have any malfunctions.

[0002] The object of the invention is to disclose a method forprotecting an electric motor and/or its triggering against overload, anda circuit arrangement suitable for performing such a method, with whicheven in the event of malfunctions in the trigger circuit itself,protection of the electric motor and its power supply against overloadis assured. This is attained by means of a method defined by claim 1 anda circuit arrangement defined by claim 8. In particular, it is thusassured that in the case of so-called emergency operation of the motor,that is, when its regular trigger circuit has failed or been put out ofoperation, compensation for an excessive battery voltage and an overloadon the drive mechanism caused by it is assured.

[0003] Features and refinements of the invention will become apparentfrom the dependent claims and the description of the exemplaryembodiment for protecting a direct-current fan motor of a motor vehiclehaving a 12-volt direct current supply to the on-board electricalsystem. The method serves to assure emergency operation for an electricmotor, which in the case of a fan motor for motor vehicles must beassured on the one hand, yet on the other must not itself be allowed tocause damage. The focus is particularly on interrupting the trigger lineof the radiator fan end stage, that is, on the proper reception of thetrigger signals from the end stage, and on monitoring the supply voltagesupplied, or in other words in the case of the motor vehicle, thebattery voltage delivered via the ignition starting switch.

[0004] One advantageous method is embodied such that in the triggercircuit, at least one overvoltage threshold is defined, which when it isreached or exceeded leads to a reduction in the power supply to themotor, or the shutting off of the motor. Preferably, the method and thecircuit arrangement are designed such that below a first overvoltagethreshold, a power corresponding to the range between 30% and 100% ofthe battery voltage is supplied to the motor. Until the firstovervoltage threshold is reached, a power corresponding to the fullbattery voltage can be applied to the motor, but selectively, the powercan also be already reduced, for instance to half the power value, uponactivation of the emergency-operation controller or upon reaching thebattery voltage. The full battery voltage, in pulse width modulation,corresponds to triggering at a 100% duty cycle, while at half thebattery voltage, the duty cycle is 50%. Preferably, however, twoovervoltage thresholds are specified, and below the first threshold, theelectric motor is supplied with a power corresponding to the fullbattery voltage, while in the range between the first threshold and thesecond threshold, a power reduced preferably to 50% of the batteryvoltage is fed into the motor. When the second overvoltage threshold isreached, the motor is then switched off, to prevent an overload on themotor itself and/or on its end stage.

[0005] A voltage monitoring circuit that is part of the motor triggeringwith regulated emergency operation according to the invention has acomparator assembly with a number of comparators corresponding to thenumber of overvoltage thresholds; the comparators are supplied on theone hand with reference voltages corresponding to the particular definedovervoltage thresholds and as a tripping variable, a reference valuecorresponding to the actual battery voltage. Thus in an especiallysimple and secure way, the prevention of an overload on the drivemechanism is assured, since the control is based on the definitivevariables for the overload themselves.

[0006] An especially suitable circuit arrangement for protecting anelectric motor and/or its triggering against overload in theemergency-operation mode, in particular for protecting a direct-currentfan motor of a motor vehicle, uses a motor end stage with pulse-widthmodulated triggering and is distinguished in that for protection againstoverload in the emergency-operation mode, in which the supplied powercan no longer be assured by the PWM trigger circuit that is operative innormal operation, the electric motor is additionally provided with amicrocontroller, which precedes the end stage and has an associatedcomparator assembly for monitoring the supply voltage. Thisemergency-operation controller becomes operative particularly in theevent of an interruption or in the absence of a feed-in of the set-pointvalue signal of the pulse width modulation and/or of the supply voltageswitched to the motor end stage, because now the motor end stage issupplied with a control voltage corresponding to one or more overvoltagethresholds, which are monitored and indicated to the microcontroller.

[0007] Further details and advantageous features of the invention willbecome apparent from the description of the exemplary embodiment.

[0008] In the drawings,

[0009]FIG. 1 shows the course of the duty cycle TV of the voltagedelivered to the electric motor, as a function of the variable batteryvoltage U_(bat); and

[0010]FIG. 2 shows a circuit arrangement of an overload protector of theinvention for the emergency-operation mode of a radiator fan for a motorvehicle.

[0011] In FIG. 1, the duty cycle TV of the pulsed supply voltage isplotted as a percentage over the battery voltage U_(bat). To simplifythe drawing, only the values for 50% and 100% are shown for the dutycycle TV. For the battery voltage, values are marked from 13 V to 16 V.A battery voltage of 13 V corresponds to the normal supply voltage; afirst overvoltage threshold U_(s1) is defined at 14 V, and a secondovervoltage threshold U_(s2) is defined at 16 V.

[0012] In the drawing in FIG. 1, various courses of the duty cycle TV,which corresponds to the mean value of the supplied voltage to the drivemechanism, are shown. A dot-dashed line represents the usual course ofthe pulse width modulation; below the normal battery voltage of 13 V,the full voltage is applied in accordance with a duty cycle of 100%,while above this value the duty cycle is reduced—linearly, in the caseillustrated—and the value of zero is reached at a battery voltage of 16V. This kind of control of the supply voltage is described in DE 199 44194 A referred at the outset here.

[0013] In the emergency-operation mode, the case in which theabove-described regulation of the supply voltage, shown in dot-dashedlines in FIG. 1, fails must be taken into account in particular. Sincein the normal situation the electric motor is designed for a supplyvoltage of 13 V, if a supply voltage of 16 V were applied in the eventof failure of the regulation, the motor and its end stage would beoverloaded in every case, which is prevented in the control according tothe invention here.

[0014] In FIG. 1, a solid line represents a voltage course in which,after the failure of the PWM triggering up to a value of 14 V, the fullbattery voltage is applied to the motor. In the normal situation, thisslight overload would still be tolerated without damage by a radiatorfan motor for motor vehicles and by its end stage. After the firstovervoltage threshold U_(s1) of 14 V is reached, the duty cycle is thenset back to 50%, so that now only half the battery voltage is applied tothe motor. This still makes an adequate emergency-operation power to themotor possible. Not until the second overvoltage threshold U_(s2) of 16V is reached is the supply voltage then switched off.

[0015] A further alternative is represented in FIG. 1 by a dashed line.This illustrates the possibility of setting the supply voltage back tohalf directly upon the occurrence of the emergency-operation situation,corresponding to a duty cycle of 50%, and of maintaining that valueuntil the shutoff at the second overvoltage threshold U_(s2). It iscertainly possible to provide still additional thresholds. For instance,it would be conceivable to define a first overvoltage threshold alreadyat the rated value for the battery voltage of 13 V, and to reduce thevoltage when this value is reached. It would fundamentally also bepossible in the emergency-operation mode to perform a regulatedreduction of the duty cycle TV, for instance corresponding to the courseshown in dot-dashed lines for nonfaulty operation.

[0016]FIG. 2 shows a circuit arrangement for protecting a direct-currentfan motor of a motor vehicle and its trigger circuit 12 againstoverload. The drawing shows an electric motor and fan, marked 10, whichis supplied by the trigger circuit 12 with a pulse-width modulatedvoltage PWM_(mot). The trigger circuit 12 essentially comprises acomparator assembly 12, a microcontroller 16, and a motor end stage 22.The triggering in the normal operating mode of the electric motor 12 iseffected via a motor vehicle control unit 18, typically called aMotronic, which supplies the microcontroller 16 with the set-point valuefor the PWM. The microcontroller 16 is also connected to the output ofthe comparator assembly 14 and to the switched battery voltage U₁₅downstream of the ignition switch. The output of the microcontroller 16with the pulse-width modulated control signal PWM is connected to themotor end stage 22, which is also connected to the vehicle battery, notshown, and which from there receives the battery voltage U_(bat). Thepulse-width modulated supply voltage PWM_(mot) is applied to theelectric motor 10 at the output of the motor end stage 22.

[0017] In the exemplary embodiment, the comparator assembly 14 comprisestwo comparators 24 and 26, at each of whose noninverting inputs areference value U_(ref) derived from the actual battery voltage isapplied. Each reference value is compared with the overvoltagethresholds U_(s1) and U_(s2), respectively, already explained inconjunction with FIG. 1, which for a 12-volt on-board electrical systemof a motor vehicle are expediently defined at values of 14 V for U_(s1)and 16 V for U_(s2), as limit values for a normal battery voltage of 13V as the supply voltage to the electric motor 10.

[0018] The output signal PWM_(sol1) of the Motronic 18, in particular,is monitored as a malfunction variable for the emergency-operation modeof the electric motor 10. For instance, if the communication between theMotronic 18 and the microcontroller 16 is interrupted, an overload canoccur at the electric motor 10 or at its end stage 22, if at the sametime, upon a malfunction in the pulse width modulation of the triggersignal, an excessive increase in the battery voltage U_(bat) occurs.

[0019] Another reason for activating the emergency-operation controllercan be interruption of the switched battery voltage U₁₅ downstream ofthe ignition switch of the motor vehicle; this malfunction can occursimultaneously with or separately from a malfunction in the triggersignal of the Motronic 18.

[0020] If a malfunction occurs, then a signal corresponding to aovervoltage threshold U_(s1) of 14 V is present at the inverting inputof the comparator 24 in the comparator assembly 14, while an inputsignal corresponding to the overvoltage threshold U_(s2) of 16 V ispresent at the inverting input of the comparator 26. Both noninvertinginputs of the comparators 24 and 26 are acted upon by a referencevoltage U_(ref), whose magnitude is determined by the actual batteryvoltage U_(bat).

[0021] In an emergency-operation program represented by the solid linein FIG. 1, the comparator assembly 14 and with it theemergency-operation controller are inactive as long as the batteryvoltage does not exceed a value of 14 V corresponding to the thresholdU_(s1). Up to that time, the duty cycle TV is 100%, and the batteryvoltage is applied to its full extent to the electric motor 10 via themotor end stage 22. When the overvoltage threshold U_(s1) of 14 V isreached, the duty cycle TV is lowered to a value of 50%, and half thebattery voltage reaches the motor, until the overvoltage thresholdU_(s2) of 16 V is reached, and the supply of current to the electricmotor 10 is switched off. The same is correspondingly true for the othercourses shown in FIG. 1 for a battery voltage that is reduced in theemergency-operation mode. In the case of the dashed line, immediatelyupon the occurrence of a malfunction at the malfunctioning sites 28and/or 30, the supply voltage is halved by a reduction in the duty cycleto 50%, and it continues to be applied at that level to the electricmotor 10 until the overvoltage threshold U_(s2) of 16 V is reached andthe motor is switched off.

[0022] As represented by the dot-dashed line in FIG. 1, a regulatedsupply can also be assured in the emergency-operation mode. In addition,additional overvoltage thresholds can be defined, and intermediatestages in the reduction in the supply voltage can be achieved. Themonitoring function in the trigger circuit 12 by the microcontroller 16is not limited to complete failure of the PWM_(sol1) signal or of theswitched supply voltage U₁₅; instead, and in particular, monitoring ofthe variables PWM_(sol1) and U₁₅ for plausibility is also done in themicrocontroller 16. For the variables U₁₅ and PWM_(sol1), limit valuesof 10% and 90%, for instance, respectively, can be defined, which whenthey are undershot or exceeded indicate failure.

[0023] The pulse frequency of the pulse width modulation of this kind oftrigger circuit 12 is typically in the range between 100 Hz and 300 Hz,and this pulse frequency is also monitored; once again, slightfluctuations are not identified as a malfunction. Finally, instead ofthe Motronic 18, any other closed- or open-loop control unit can be usedfor the pulse width modulation, and in particular can also be integratedwith the microcontroller 16. What is essential that if the normalclosed- or open-loop control fails with a simultaneous occurrence of anelevated battery voltage, the electric motor 10 or its end stage 22 willnot be overloaded; instead, in that case the emergency-operationcontroller intervenes and maintains further operation of the electricmotor 10 within specified limits.

[0024] Thus the circuit arrangement of the invention makes an internalvoltage compensation within the motor trigger circuit possible if themotor control unit, for instance in the form of the Motronic 18,normally located upstream of the trigger circuit fails; this controlunit itself, if there is an interruption in the trigger line in theevent of an elevated battery voltage, cannot prevent an overload.

1. A method for protecting an electric motor (10) and/or its triggercircuit (12) against overload, particularly for protecting adirect-current fan motor of a motor vehicle, wherein the electric motoris assigned a motor end stage for pulse-width modulated control (PWM) ofthe power supplied and is also assigned a controller for limiting themotor power on the occurrence of an elevated supply voltage,characterized in that the overload protection becomes operative inemergency operation of the electric motor (10), by activation of anemergency-operation controller (14, 16) upon the occurrence of amalfunction (28, 30) in the triggering (8) of the motor end stage, andthe electric motor (10) is supplied with a defined emergency-operationpower, which below a first overvoltage threshold (U_(s1)) of the supplyvoltage has a specified value or course and is reduced or switched offif one or more overvoltage thresholds (U_(s1), U_(s2)) are exceeded. 2.The method of claim 1, characterized in that the electric motor (10) issupplied, below the first overvoltage threshold (U_(s1)), with a powercorresponding to the range between 30% and 100% of the battery voltage(U_(bat)), and in particular a power corresponding to the full batteryvoltage (U_(bat)).
 3. The method of claim 1 or 2, characterized in thattwo overvoltage thresholds (U_(s1), U_(s2)) are specified, and betweenthe first threshold (U_(s2)) and the second threshold (U_(s2)), theelectric motor (10) is supplied with a power corresponding to the rangebetween 30% and 70% of the battery voltage (U_(bat)), and preferably apower corresponding to half the battery voltage.
 4. The method of one ofthe foregoing claims, characterized in that the electric motor (10) isswitched off when the second overvoltage threshold (U_(s2)) is reached.5. The method of one of the foregoing claims for use in a 12-volton-board electrical system of a motor vehicle, characterized in that thefirst overvoltage threshold (U_(s1)) is fixed at a value of 14 V.
 6. Themethod of one of the foregoing claims for use in a 12-volt on-boardelectrical system of a motor vehicle, characterized in that the secondovervoltage threshold (U_(s2)) is fixed at a value of 16 V.
 7. Themethod of one of the foregoing claims, characterized in that the powerof the electric motor (10) in the emergency-operation mode is regulatedby means of a voltage monitoring circuit with a comparator assembly(14), to which a reference voltage (U_(ref)) corresponding to thebattery voltage (U_(bat)) is supplied as a reference value for theovervoltage thresholds (U_(s1), U_(s2)).
 8. A circuit arrangement forprotecting an electric motor and/or its trigger circuit (12) againstoverload, in particular for protecting a direct-current fan motor of amotor vehicle, having a motor end stage for pulse-width modulatedcontrol (PWM) of the power supplied to the electric motor, and having acontroller for limiting the motor power on the occurrence of an elevatedsupply voltage, in particular for performing the method of one of theforegoing claims, characterized in that for the overload protection inthe emergency-operation mode of the electric motor (10), its triggercircuit (12) is embodied as an emergency-operation controller (14, 16,22), which in addition to a PWM set-point value transducer (18) and themotor end stage (22) has a microcontroller (16), which precedes themotor end stage and which upon an interruption or a defective input ofthe set-point value signal of the pulse width modulation (PWM_(sol1))and/or of the switched supply voltage (U₁₅) supplies a control voltage(PWM) to the motor end stage (22), corresponding to the specification ofat least one overvoltage threshold (U_(s1), U_(s2)) to themicrocontroller.
 9. The circuit arrangement of claim 8, characterized inthat the emergency-operation controller (14, 16, 22) has a comparatorassembly (14), to which on the one hand a reference voltage (U_(ref))corresponding to the battery voltage (U_(bat)) and on the other, as acomparison value, at least one overvoltage threshold (U_(s1), U_(s2))above the normal battery voltage (U_(bat)) are specified.
 10. Thecircuit arrangement of claim 8 or 9 for use in a 12-volt on-boardelectrical system of a motor vehicle, characterized in that thecomparator assembly (14) has two comparators (24, 26), to which on theone hand a reference value (U_(ref)) derived from the actual batteryvoltage (U_(bat)) and on the other overvoltage thresholds ofapproximately 14 V (U_(s1)) and approximately 16 V (U_(s2)),respectively, above the normal battery voltage (U_(bat)) ofapproximately 13 V, are specified.